RoboVoice Speaker is a text to speech service and you can add it to your website at ease. The whole system is powered by Microsoft Translator, so a lot of speaking languages are available for your users. You will only need a Bing AppID to work with the library.
Go to Bing Developer Center in order to get it. Some of the features are: Easy text-to-speech API, Automatically language detection, Playing text parts one by one, No dependences from extra frameworks, compatible with Internet Explorer (IE6+), Chrome, FireFox, Safari, Opera and more!